Comparing size evaluation methods for acoustic neuroma after stereotactic radiosurgery

Purpose. Acoustic neuroma tumor size may be evaluated using several methods. Here we investigate the variations among measuring techniques. Materials and methods. A retrospective analysis of pre- and posttreatment magnetic resonance (MR) scans was performed on 15 acoustic neuroma patients with a history of stereotactic radiosurgery who had been followed for more than 2 years. Tumor size was measured on each MR scan using three methods, where the extracanalicular (EX) and intracanalicular (IN) portions were measured separately. We collected data on the largest diameter (M1), the square root of the product of the maximum anteroposterior and mediolateral diameter (M2), and the average for the maximum anteroposterior, mediolateral, and superoinferior diameters (M3). Size differences between follow-up MR scans separated by more than 2 years were calculated for each method, and we evaluated whether the tumors progressed, remained stable, or regressed. Results. A total of 154 follow-up pairs of EX and 115 follow-up pairs of IN showed a statistically significant difference for the number of each category among the three methods (P = 0.03, P < 0.01, respectively). The greatest category agreement was observed between the M2 and M3 methods. Conclusion. A significant difference between the tumor size measuring methods was observed. To strengthen specificity when evaluating tumor size difference, a measuring method using two or more parameters is recommended. © 2007 Japan Radiological Society.